The Circadian Case for Standard Time
As the Earth rotates on its tilted axis around the sun, the timing of sun exposure varies based on your latitude. During the solstices in the summer and winter, the duration of light exposure is maximized and minimized, respectively. Similarly, during the spring and autumn, equinoxes occur, in which the day and night are the same length at nearly all latitudes.
Interestingly, biology has latched onto this phenomenon and used it as a regulator for many biological clocks. Nearly all living organisms have developed biological clocks to regulate physiology and behavioral timings. In humans, the central clock lives deep in the brain in the suprachiasmatic nucleus of the hypothalamus and is mainly regulated by neural inputs resulting from ocular light exposure. This clock coordinates signals that regulate the timing of the peripheral clocks. Well-regulated clocks allow us to take advantage of predictive physiology.
Getting tired just before bed or getting hungry just before lunch are examples of what might be noticeable if your tapping into the power of predictive physiology. Coordinated efforts of multiple systems behind the scenes produce these noticeable effects. These systems however can also be responsive (rather than predictive). For example your digestion can also kick into gear after eating rather than in anticipation inevitably delaying the response in comparison.
Back to the regulation of predictive physiology, it comes down to the maintenance of the circadian rhythm through its central clock. There are three main factors that the central clock takes into account during regulation: (1) current timing of the central clock, (2) the illuminance (brightness) of the light hitting your eyes, and (3) the duration of that light exposure.
The timing of the central clock is important since light exposure is a regulatory signal rather than a driving signal. Absent of light exposure, the clock still persists in ticking, as is the nature of an endogenous rhythm, but the typical period of this clock is about 24.2 hours rather than the 24 hours that the outer world operates on. To stay aligned with the outer world, each day humans must get light exposure at the correct time relative to their central clock (hence factor 1). This is because the exact same light exposure delivered at different times can shorten the period, lengthen the period, or have no effect. During the early clock times, there is period shortening behavior, which is why it is recommended to get morning light exposure to keep in sync with the outer world. During middle clock times, there is little effect on phase. In later clock times, there is period lengthening behavior, which is why evening light can shift your clock so you want to go to bed later the next day. Midnight (time-zero) of the central clock is not perfectly matched with midnight of the outer world. It’s usually shifted so that your body’s midnight, where it switched from period lengthening to period shortening behavior, is around 2 hours before you typically wake up in the morning.
In addition to this, the brighter the light, the more your clock will be shifted. Indoor light is generally around 100 – 500 lux, whereas outdoor light is around 10k – 100k lux. Getting outdoors for morning light can be important due to this difference and its impact on period shortening. Lastly, the duration of light exposure matters. Generally, the longer you’re exposed to light, the more it impacts your system, but these effects are non-linear and depend on the timing of the central clock.
While the system regulating the clock is complicated, there is little widespread education on the impact of the light diet on human health. Most people never pay attention to their light diet, nor think of it as something that can be managed. This becomes increasingly important with our modern ability to control our light environment and change it on a whim.
It also becomes important in policy decisions that may impact regulation of the circadian system on a population level. While we can’t control the tilt of the earth and seasonal changes in lighting, we can select policies that might encourage circadian health. With the major policy debate on whether to keep or remove daylight savings, we must consider both the acute and long-term consequences of our options.
The first consideration is the acute consequences of a rapid, population level shift in timing. It is generally more difficult to “spring ahead” and realign your physiological and behavioral timing to one hour earlier. The shifting processes can be particularly disruptive to vulnerable populations, and there have been noted increases in cardiovascular medical events and car crashes as well as sleep disturbance and disrupted mood following the shift. Since shifting in itself can be disruptive, the first recommendation would be to avoid shifting.
The next consideration is the long-term consequence of societal timing. During Standard Time, the activities of the world generally start later in the day relative to Daylight Savings. Standard Time year-round gives people greater opportunity to get the morning sunlight necessary to shorten the period of their central circadian clock from 24.2 hours to 24 hours to maintain synchrony with the outer world. Daylight savings year-round, on the other hand, might limit the ability to get sufficient morning light exposure, especially for those who work indoors during normal work hours. While the appeal of evening sunlight under Daylight Savings Time is understandable, this is largely a seasonal phenomenon tied to longer summer days. In fall and winter, when daylight hours are shorter regardless of time policy, prioritizing morning light through Standard Time becomes even more critical for maintaining circadian health. From a circadian prospective, the second recommendation would be to favor Standard Time year-round.
Overall, the circadian system and its regulation are crucial for optimizing health and well-being. By understanding and aligning with the science of circadian rhythms, we can make informed choices that benefit both individuals and society. Policies, such as the decision to adopt year-round Standard Time, have the potential to promote healthier circadian rhythms on a population level. Prioritizing circadian health isn't just about convenience—it's about fostering a world where our biological clocks are in harmony with the natural cycles that sustain us.
